1. Field of the Invention
This invention relates in general to a system for identifying a number encoded into a section of drill pipe as the drill rig raises and lowers the string of drill pipe, and in particular to such a system employing surface acoustic wave devices.
2. Description of the Prior Art
In drilling wells, a string of drill pipe is employed to rotate a drill bit. The string is made up of sections of drill pipe about thirty feet in length and screwed together. It is very important that fatigue does not develop in any of the drill pipe sections to such extent that the drill pipe would part. Retrieving the section of drill pipe which has been lost in the hole can be very expensive.
Fatigue is more likely to occur in deviated wells which are commonly drilled in offshore locations. Deviated wells result in bending of the pipe as it is rotated, placing greater stress than in a straight well. To avoid overusing the string in deviated wells, the operator must keep track of the footage drilled, then retire the string before the danger of fatigue occurs. Keeping track of the string is not an easy task since a drill string can be used for drilling many wells. Also, the sections of the string may be interchanged with sections from other strings. In addition, some of the sections may have been primarily located in portions of the well that curve more sharply than in other areas of the well, thus subjecting them to greater stress than the sections which have been run primarily in straight portions of the hole.
No means is currently employed for identifying individual drill pipe sections. The drill pipe sections are pulled in and out of the well too quickly to be manually identified by anything such as a serial number stamped on the drill pipe. U.S. Pat. Nos. 4,061,967, James R. Hall, Dec. 6, 1977 and 4,202,490, Walter A. Gunkel et al., May 13, 1980, disclose systems for automatically detecting an identification number on each drill pipe section as the drill pipe moves through the rig floor. However, none of these systems are commercially available.
In recent years, surface acoustic wave technology, normally called SAW technology, has been used to identify various objects. This technology includes a SAW tag which has a chip and an antenna. The chip consists of a base substrate of piezoelectric material onto which an aluminum film pattern of interdigitated transducers is deposited. The transducers resemble, under a microscope, opposing combs with interlaced teeth. Unique coding of the device is achieved photographically by modifying the reflector mechanism.
A transmitter/receiver sends out a radio frequency signal which is picked up by the antenna of the SAW tag. The electromagnetic energy received by the SAW tag antenna is converted into acoustic sound energy in the form of ultrasonic surface acoustic waves which propagate along the SAW substrate of the chip and are reflected by a reflecting structure in the acoustic path. The reflectors modify the phase or amplitude of the ultrasonic waves to give the device its unique code.
The ultrasonic waves are then converted by the transducers on the SAW tag back into electromagnetic energy or "echoes", which are reradiated back to the transmitter and receiver. The reradiated signal has a distinctive code which has been predetermined by the photographic trimming of the reflector structure. The signal received by the transmitter/receiver is processed and converted into a digital signal which is stored in a memory unit.
The SAW technology has been proposed for various applications used in identifying devices. The use of SAW technology has not before been proposed for identifying drill pipe sections. The use in drill pipe presents a formidable challenge. The identification tag must be mounted to the drill pipe such that it could handle high temperatures and high pressures in the well bore, and for extended periods of time exceeding several hundred hours. Also, the drill pipe is subjected to shock and vibration which can affect the reliability of such a device.